The carbon content of fly ash is the major determinant of combustion efficiency for coal-fire, d boilers. Carbon content is presently measured offline by a Loss-On-Ignition CLOI) test. The LOI test is typically performed by collecting a sample of fly ash from the boiler, weighing the sample, heating the sample to a temperature sufficient to drive moisture from the sample, reweighing the sample to determine the moisture content in the sample when collected, reheating the sample in an air stream to a temperature sufficient to oxidize the carbon in the sample to carbon dioxide, and weighing the remaining sample to determine the carbon content by the difference in weight between the sample prior to oxidation of the carbon and after the oxidation step.
There are a number of disadvantages associated with the typical LOI test. One disadvantage is the tedious and time consuming steps necessary to treat the sample to provide the relevant dam. Furthermore, the LOI test may introduce inaccuracies into the data if mineral matter, such as limestone or other substances, are present which exhibit weight changes upon heating in addition to the changes caused by carbon oxidation.
Attempts have been made to provide monitors based on the photoacoustic effects which occur when energy is directed at the fly ash in the flue gas of a coal-fired furnace. Briefly, the photoacoustic effect is caused by the absorption of energy by the carbon in the fly ash. After absorbing-energy, the carbon produces a thermal wave which, in turn, produces a minute acoustical signal generated at a frequency equal to the modulation frequency of the energy
Previous attempts at producing carbon monitors based on the photoacoustic effect employed optical energy as the excitation source. The disadvantage with such systems is, however, that the response is typically dependent upon the size distribution of the carbon particles in the sample unless the wavelength of the excitation radiation is much larger than the largest particle in the sample being tested. Fly ash typically includes carbon particles ranging in size from 10 microns to 100 microns in diameter. Optical radiation, however, typically has a wavelength of less than 0.7 .mu.m. As a result, the photoacoustic response of fly ash samples to laser radiation excitation is typically dependent upon the size of the carbon particles in the sample.